Method for desalting and dewatering crude coal tar



sept. 29, 1964 J. M. MULLER 3,151,055

METHOD FOR DESALTING AND DEWATERING CRUDE COAL TAR Filed March 2, 1961 mA/vsu/ "En HOT IVA TER /lVI/E/VTU/i JOSEPH M. MULLER "gnu/@M Attorney United States Patent O 3,151,055' METHOD FR DESALTNG AND DEWATERING CRUDE CGAL TAR Joseph M. Mulier, Portage, Ind., assigner to United States Steel Corporation, a ycorporation of New Jersey Filed Mar. 2, 1961, Ser. No. 92,945 7 Claims. (Cl. 208-39) This invention relates generally to the recovery of crude coal tar from the destructive distillation of bituminous coal, and more particularly to a method for dewatering and desalting, i.e. reducing water and water-soluble salts in the crude coal tar thus produced, prior to tar distillation.

In conventional coke-plant practice, the foul gas from the coke ovens is cooled by spraying with flushing liquor which, along with the condensible tars and other compounds, flows to a decanter for separation of the liquor `from the condensed tar. The flushing liquor is recycled for further gas cooling while the condensed tar is pumped to a wet-tar storage tank. The gas and remaining vapors are further cooled in primary coolers, `the condensate comprising tar and liquor 'being decanted and separated. The tar is pumped to a wet-tar storage tank. Primarycooler tar is less viscous than flushing-liquor tar. These tars may be combined or separately stored for different end uses. In the practice under consideration they are combined in a wet-tar storage tank for subsequent distillation.

Production of crude coal tar as hereinabove described is effected by condensation of a vapor to a liquid. The condensation results in entrainment of liquor with the tar which does not separate completely in the described decantations, resulting in so-called wet tar. The liquor contains a number of dissolved salts, and by desalting is meant a reduction in these water-soluble salts. Ammonium chloride is one of these salts and is important since in subsequent tar distillation, it is deleterious to steel equipment, especially at the higher temperatures attained in distillation equipment. The desalting operation to be described and claimed involves the dilution of and reduction in ammonium chloride principally, although it is to be understood that there will be a reduction in the other soluble salts present in the wet-tarliquor.

In conventional practice the entrained liquor is separated by further decantation with or without heating, or by the use of a centrifuge. While these means serve to reduce the entrained liquor or moisture content they do not reduce the salt content of the residual liquor.

F I have discovered that numerous advantages over con- Ventional practice are obtained by admixing with wet tar, water at a temperature higher than that of the tar, heating the admixture to at least about 100 C. and allowing the heated admixture to separate into liquor and dewatered-tar layers in a substantially full, bottom-heated stor- `age tank with gradual reduction in temperature from top to bottom of the tank. Chemical additions may further reduce the ammonium-chloride content of the dewatered tar.

A complete understanding of the invention may be obtained from the following detailed description and explanation which refer to the accompanying drawing illustrating the present preferred practice, The single iigure of the drawing is a diagrammatic representation of a system for carrying out the method of my invention.

Referring now in detail to the drawing, wet coal tar is introduced by a pipe 10 from decanters (not shown) to a wet-tar storage tank 12. The entrained-liquor or moisture content of the tar may vary widely, but is usually about 5% to 12% by volume of the tar. The ammonium-chloride content of the wet tar may vary widely the more stable sodium chloride.

Patented Sept. 29, 1904 ice but is usually about .010% to .030% by weight ofthe wet tar. Steam-heating coil 14 serves to caintain the effluent-tar temperature between about C. and 95 C. Tar issuing from the bottom of tank 12 is pumped by pump '16 and pipe 18 to a heat exchanger 20. Waterat a temperature preferably `hotter than the tar, usually at a temperature between about C. and 97 C., is injected through pipe 22 into the tantraversing pipe 18. Control valve 24 supplies sufficient water to raise the total liquor or moisture content of the tar to between about 10% and 15% by volume of the tar, thereby diluting the salt contained in the liquor. l i

The admixture flowing through heat exchanger 20 is Vheated by steam -from pipe 2 6 to at least about 100 C.,

preferably to between about 102 C. and 110 C., thereby reducing the `viscosity of the tarcontent ofsaid admixture. A thermocouple in pipe 28 actuates a temperature controller 30 which in turn controls steam flow by means of valve 32. The heated admixture -in pipe 28 ows to the top of a dewatered-tar storage tank 34. Contrary to conventional practice of maintaining the tar hotter in the bottom of the tank than at the top, the tar in tank 34 is at least equal in temperature to but preferably hotter at the top, because of said heated admixture flowing thereto, whereas the bottom-tar temperature ismaintained between about C. and 98 C. by steam coil 36. Operating in this fashion to maintain a hotter, lowerviscosity tar in the upperv portion of the tar layer, it has been found that the entrained liquor separates more efficiently, despite the deliberate increase in the liquor or moisture content.

The combined liquor layer 38 and dewatered-tar layer 40 substantially fili tank `34 at all times. Float 42 acting vthrough transmitter 44 and cable 46, operates valve 48 to control wet-tar input substantially equal to dewateredtar flowto the sti-lls. Liquor'flows from tankf34 through valve 50 which may be left open, but preferably is manually operated periodically to maintain a rliquor layer between about 3 inches and 12 inches in a conventional 500,000-gallon tar-storage tank. Transmitter' 44 is manually adjusted to raise'the tank level and to overflow liqf uor through valve 50.

Dewatered tar, its liquor content reduced to below about 3% and its ammonium-chloride content to below about .006% by weight of the tar, ows from the bottom of tank 34 through pipe 52 and pump 54`to a tarstill (not shown). A water solution of sodium carbonate may be used to react with the ammonium chloride to form The solution in tank 55 flows through pipe 58 pump 60 and pipe `62 to pipe 52. Admixture ahead of the pump provides sufficient turbulence for effective reaction with the ammonium chloride.

Itwill be apparent that the tar tanks, pumps, control- 1ers and heat exchanger may be of any design known to effect the desired result. In particular, heat exchanger 20 is of the tube type, the tar owing through the tubes.

A specific example of the practice of the invention, giving quantities involved, is set forth below.

ln a typical coke plant producing about 150,000 gallons per day of combined flushing-liquor and primaryacooler tars, the tars from the decanters are stored in one or more large, eg., 500,000-gallon Wet-tar storage tanks 12. Coil 14 maintains the tar between about 75 C. and 80 C. The tar containing about 8% by volume of entrained liquor and having an ammonium-chloride content of about .015% by weight of wet tar is admixed'with water between about 80 C. and 90 C. in a quantity sufficient to raise the liquor content to about 15 The .admixtureis thereafter heated in heat exchanger 20 to between about 102 C. and 110 C. and conducted to dewatered-tar storage tank 34 of 500,000-gallon capacity, the combined liquor and dewatered-tar volume being about 465,000 gallons. The liquor layer, about 3% of the total depth, is controlled between about 3 inches and 9 inches, liquor being removed from the tank 34 at 8 hour intervals by raising the tank level through manual operation of transmitter 44. Coil 36 maintains the tar temperature in the bottom of the tank between about 95 C. and 98 C. Dewatered tar flowing from the bottom of tank 34 has a liquor content of about 2.5% and an ammoniumchloride contenty of about .005%, a reduction of about 67% of the ammonium-chloride content of the tar in tank 12. Reaction ofthe ammonium chloride to form sodium chlorideis effected with a 12% water solution of sodium carbonate, an excess of about 60% over theoretical for the reaction being found to produce the desired results.

While the above example illustrates a preferred method of operation, other conditions ofoperation may be used without departing from the spirit of the invention. The wet-coal tar with its entrained liquor will vary in temperature between about 75 C. and 95 C. according to the season and the liquor content. The temperature will normally be maintained higher in colder weather and as the liquor content increases, to aid in separation of the liquor. It is preferable for good mixing to add water at a temperature higher than that of the tar, the water temperature being at least about 2 C. hotter than the tar. Water admixture to a total of about 10% to 15% liquor is a good .practical limit where an ammonium-chloride removal problem exists. The total liquor content may be increased to about 18%, but it should be appreciated that, with further increase in the liquor content, the retention time in the dewatered-tar storage tank must be increased for good liquor separation. Additional liquor will ash-oi too much steam in this tank. At below about 10% total liquor, less reduction of ammonium chloride will take place than is possible with my method. It has been found that the viscosity of a mixture of iush- `ing-liquor and primary-cooler tars heated to between limits of 100 C. and 110 C. to which the admixture to the dewatered-tar tank is heated, a higher Viscosity flushing-liquor tar, when treated alone, should be heated to the lhigh side of this range, and a lower-viscosity primarycooler tar to the low side of the range. Under the above temperature conditions and operating with a substantially full tank and substantial balance between input and output of the dewatered-tar tank, sucient retention time is afforded the admixture for separation of liquor and tar. It is obvious from the principle of this invention that a longer retention time, if required, may be achieved with a larger tank or a reduced throughput. As for maintenance of the 3 to 12 liquor layer, it is preferred to operate on the low side of the range. At above 12, the incoming admixture may carry down too much liquor.

The invention is characterized by several distinct advantages. In the iirst place, the separation of liquor and tar is a relatively simple, inexpensive and eicient process as compared to the former practice of centrifuge separation of liquor and tar which involves additional maintenance expense. Wet tar with a liquor content of 5% to 12% and higher can be reduced consistently to between about 2.0% and 3.5% liquor content.

Secondly, the addition to the wet tar of water at a temperature higher than that of the tar dilutes the ammonum-chloride concentration of the wet tar. Hence, each unit of liquor remaining in the tar contains less am monium chloride than the same unit of liquor in the wet tar. With the eiiicient liquor reduction the overall ammonium-chloride content is consistently reduced up to 70%. Hence, the tar may be distilled with a greatly reduced deleterious effect on the distillation equipment.

Thirdly, the process can be easily varied to handle the higher-viscosity flushing-liquor tar and the lower-Viscosity primary-cooler tar. Utilizing the principle of this in vention, tars with viscosities outside these limits may be processed.

Fourtlily, residual ammonium chloride may be reacted with sodium carbonate to form sodium chloride. The economics thereof is improved because of the reduction in ammonium-chloride content. This reaction converts unstable ammonium chloride to stable sodium chloride, which is eliminated with the pitch.

Fifthly, an operator may be faced with a sudden and even prolonged period of excessively high liquor content in his wet-tar storage tank. In such an emergency he may reduce or even eliminate his hot-water addition,A eiiect high liquor separation in the dewatered-tar storage tank with increased retention time and neutralize the additional ammonium chloride by increasing the sodium-carbonate addition.

Finally, an operator may not have a chloride problem when using my process with the hot-water addition. For example, the end use may not require reduction of the chlorides, or the chloride content may be sufficiently low that further reduction through hot-water additionV is unnecessary. Under these conditions, without hot-water addition, the wet tar may be heated to between about 100 C.A and 110 C. and separated from liquor in a substantially full, dewatered-tar storage tank.

Although I have disclosed herein the preferred practice of my invention, I intend to cover as well any change or modification therein which may be made without departing trom the spirit and scope of the invention.

I claim:

l. A method of desalting and dewatering wet coal tar comprising admixing therewith Water at a temperature higher than that of the tar, raising the temperature of the admixture to between about 100 and 110 C., conducting the heated admixture to the top of a storage tank, maintaining a descending temperature gradient in said admixture from the top of said tank to the bottom of at least about 5 C. while allowing the admixture to separate, conducting separated liquor from the top of said tank and cooler dewatered tar of reduced water-soluble salt conten from the bottom of said tank.

2. A method as deiined in claim 1 characterized by admixing with wet coal tar, water that is at least about 2 C. hotter than the wet coal tar.

3. A method as defined in claim 1 characterized by admixing with wet coal tar, suiiicient water to raise the total liquor content thereof to between about 10% and 15 by volume of liquor.

4. A method as deiined in claim 1 characterized by maintaining the dewatered-tar temperature in the lower portion of said storage tank at between about C. and 98 C.

5. A method as defined in claim 1 characterized by maintaining said tank substantially full of tar and liquor, substantially balancing dewatered tar withdrawal and admixture admission to said tank and allowing liquor to accumulate in a layer to a maximum depth of about l2 inches.

6. A method of dewatering wet coal tar containing at least about 10% moisture comprising raising the temperature of said tar to between about 100 and 110 C., conducting heated tar to the top of a storage tank, maintaining a descending temperature gradient in said' admixture from the top of said tank to the bottom of at least about 5 C. while allowing tar and liquor to separate, maintaining ysaid tank substantially full of tar and liquor, conducting separated liquor from the top of said tank, maintaining dewatered tar at a temperature between about 95 C. and 98 Cyat the bottom of said tank and withdrawing it therefrom.

7. A method of desalting and dewatering wet coal tar comprising admixing with coal tar at a temperature between about 75 C. and 95 C., containing between about 5% and 12% entrained liquor by volume, said liquor containing between about 0.010% and .030% ammonium chloride by Weight of said tar, sulicient water to produce an adn'iixture containing about 15% liquor by volume, said water being at a temperature not exceeding 97 C., raising the temperature of the admixture to between about 102 C. and 110 C., conducting said heated admixture to the top of a storage tank, maintaining a descending temperature gradient in said adniixture from the top of said tank to the bottom while allowing tar and liquor to separate, maintaining said tank substantially full of tar and liquor, drawing olf dewatered tar from the bottom of the tank in substantially the volume of adrnixture admitted, maintaining said tar at a temperature between about 95 C. and 98 C., whereby it has an ammonium- 6 chloride content of no more than about .006% by weight of tar, a liquor content of less than about 3%, and drawing off liquor periodically from the top of said tank.

References Cited in the le of this patent UNITED STATES PATENTS 832,409 Oppenheimer Oct. 2, 1906 2,366,899 Hall et al Jan. 9, 1945 2,375,460 Barbre May 8, 1945 2,449,404 Miller Sept. 14, 1948 2,934,488 Auvil et al Apr. 26, 1960 2,937,130 Heller et al May 17, 1960 

1. A METHOD OF DESALTING AND DEWATERING WET COAL TAR COMPRISING ADMIXING THEREWITH WATER AT A TEMPERATURE HIGHER THAN THAT OF THE TAR, RAISING THE TEMPERATURE OF THE ADMIXTURE TO BETWEEN ABOUT 100* AND 110*C., CONDUCTING THE HEATED ADMIXTURE TO THE TOP OF A STORAGE TANK, MAINTAINING A DESCENDING TEMPERATURE GRADIENT IN SAID ADMIXTURE FROM THE TOP OF SAID TANK TO THE BOTTOM OF AT LEAST ABOUT 5*C. WHILE ALLOWING THE ADMIXTURE TO SEPARATE, CONDUCTING SEPARATED LIQUOR FROM THE TOP OF SAID TANK AND COOLER DEWATERED TAR OF REDUCED WATER-SOLUBLE SALT CONTENT FROM THE BOTTOM OF SAID TANK. 